Why Our Pupils Are Round, Cat’s Are Slit And Herbivores’ Are Rectangular?

The iris, the membrane that gives color to our eyes dilates or contracts depending on the luminosity of the environment. The space through which light penetrates, the window the iris leaves, is called the pupil.

Figure 1: Cats with a different level of pupil contraction or dilation depending on the environmental light. Cats can dilate their pupil up to 135 times more the original size. Humans can only expand it 15 times between maximum and minimum width.

The pupil presents different shapes depending on their eating habits of different animals. We can say that herbivores, generally those with daily habits and in the lower part of the food chain have horizontally elongated or rectangular pupils. Dogs, humans and the great felines have round pupils, while cats, foxes -and some reptiles- have vertical pupils.

But, how do we explain these differences? Why do they happen? What advantages do they have? Let’s look at this with some more detail:

Herbivores’ rectangular pupil:

Herbivores have a very extensive visual field since their eyes are placed to the sides of their head. They can, therefore, see both towards the front and the back. On the other hand, they always have to be alert, always paying attention to detect any suspicious movements that could mean a predator is nearby. They must have good frontal vision to escape and avoid obstacles quickly if needed.

Figure 3: Horse’s visual field. 350º vision of which 65º are binocular and

285º can be seen with each eye independently.

The rectangular pupil gives equines and ruminants a horizontal panoramic vision of what is happening on the ground -which is where predators could potentially be- allowing them to have a diaphanous vision from the front to the back of the visual.

This disposition permits a deeper horizontal vision and improves the quality of vision in that direction.

Furthermore, this ocular adaptation can prevent too much light from entering through the top which could blind the animal.

Eyes with a vertical pupil:

Cats’ eyes -or foxes’- behave in a very distinct way. Cats are nocturnal hunters, which is why their pupils must dilate so much. They must detect and catch their prey in low light conditions. On the other hand, due to their small size, in order to be successful as a predator they must wait, crouch and catch their prey by surprise after getting just a few centimeters away from it. In order to do that, it is critical that they measure distances properly and that they focus short distances very precisely.

Anatomically speaking one eye with a vertical pupil -some snakes also present this orientation- is the best to have the best capacity to measure the depth of a field.

Figure 4: Simulation with a vertical pupil camera.

Observe how the blurry objects in front of and behind the focused bird allow them

to give depth to the field in order to calculate distances. Source: Science Advance

They are able to calculate the depth of a field in two ways:

The brain measures the distance of the objects through the image that comes back to the eye (stereopsis) and creates a tridimensional image.

The animal measures the blurriness degree of the objects that are in front of and behind the focused object.

It turns out that the capacity of knowing the depth of a field and the high capacity of dilating pupils is maximized by having a vertical pupil.

Eyes with round pupil:

Large felines such as the tiger and the lion, on the contrary, present a round pupil. The reason lies, first, in the fact that they tend to hunt during times of the day when there is very bright light. On the other hand, their legs are longer than a cats’, their eyes are at a greater distance from the ground and they do not need to get as close to their prey because their jump is a lot more powerful. For that reason, even if they cannot focus on their victim very precisely like the cat can, they can correct their technique with certain margin of error due to their muscle strength, which means they do not need a vertical pupil.

Figure 5: Contrast in the shapes of feline pupils. The tiger and the lion present round pupils while cats present elongated vertical pupils.

The great canines, such as wolves, prey by exhausting their victims. Therefore, they do not really need a complex optic apparatus and their pupils are round. Furthermore, their hunting habits are diurnal which means that the round pupil covers their needs. The same happens with humans since our habits are diurnal as well.

As a result, we can conclude that vertical pupils serve as an evolutive mechanism that small nocturnal predators can hunt by surprise, ambushing a prey.

The round pupil is possessed by diurnal hunters who pursue their prey. Finally, the horizontal pupil is most common amongst herbivores who are the victims of those hunters.

As it happens with every rule, and even more when it comes to biology, there are exceptions, but as a general rule it works pretty well. So, according to a deep study carried out by investigators at Berkely and Durham it was concluded that out of 65 animals with frontal vision and predators that attack their victims by surprise, 44 of them had vertical pupils. Out of these animals, the immense majority had a height of 42cm or less, which demonstrates that small hunters have a greater necessity to focus their victims with millimetric precision, while herbivores do not need to focus precisely on close objects.

Out of the 42 herbivores studied in this project, 36 had horizontal pupils, also demonstrating this great correlation.